Dimensional measurements of importance to microlithography include feature sizes and feature placement on photomasks and wafers, overlay eccentricities, defect and particle sizes on masks and wafers, and many others. A common element is that the object sizes and required measurement uncertainties are often on the order of the wavelength of light or less. This can lead to interesting challenges for certain applications where measurement traceability is desirable. This paper will review and discuss the concepts of accuracy and traceability in microelectronic dimensional measurements, including: traceability in microlithography dimensional measurements, when is it needed, when is it not needed, the technical meaning of traceability and accuracy the ISO definitions of these and other relevant terms, the distinction between measurement error and measurement uncertainty, random and systematic measurement errors, Type A and Type B measurement uncertainty, calculation of measurement uncertainty, the measurement process for small objects, and sources of measurement uncertainty. The necessary and sufficient conditions for traceability will be outlined, and some examples will be given.